Optical method employing total internal reflection

ABSTRACT

A virtual image display system is provided which is made thinner through the use of an immersed beam splitter, and in one embodiment, total internal reflection. The display system includes an imaging surface on which a source object is formed, a first optical element having a reflective function and a magnification function, a second optical element having a magnification function and an immersed beam splitting element positioned between the first and second optical elements, the immersed beam splitting element including a beam splitter surrounded by an optically transparent material having a refractive index greater than air. An illumination source projects the source object formed at the imaging surface through the optically transparent material to the beam splitter. The beam splitter reflects the projected source object to the first optical element. The first optical element magnifies the projected source object and reflects a magnified virtual image of the projected source object to the beam splitter. The magnified virtual image traverses the beam splitter to the second optical element which magnifies the magnified virtual image to produce a compound magnified virtual image of the source object.

This application is a continuation of application Ser. No. 09/033,208,filed Mar. 2, 1998, now U.S. Pat. No. 5,892,624, which is a continuationof application Ser. No. 08/673,894, filed Jul. 2, 1996, now U.S. Pat.No. 5,771,124, which applications are incorporated herein by referencein their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention generally relates to a compact electronic display system.More specifically, the invention relates to a compact electronic displaysystem which provides a compound magnified virtual image of a sourceobject using two stages of magnification.

2. Description of Related Art

A continuing objective in the field of electronics is theminiaturization of electronic devices. Most electronic devices includesome form of display system which provides information to the user. Aselectronic devices are reduced in size, display systems are needed whichcan be incorporated into the increasingly smaller devices. It is thusimportant that the space required to house these display systems bereduced. In particular, it is desirable that the thickness of thedisplay system be reduced, the thickness of the display referring to thedimension of the display system which is perpendicular to the plane ofthe image formed by the display.

In general, the image provided by an electronic display may be either areal image or a virtual image. One approach to reducing the size of adisplay system is through the formation of a virtual image instead of areal image.

A real image refers to an image which is observed directly by theunaided human eye. A real image exists at a given location when a realimage can be observed by the unaided eye if a viewing surface ispositioned at the location. A photograph is an example of a real image.Examples of electronic displays which provide real images include liquidcrystal displays, CRT monitors, and projection screens. Compactelectronic devices, because of their small size, have a limited surfacearea on which to provide a real image. Since the amount of detail thatthe human eye can resolve per unit area is limited, devices whichprovide a real image are only able to provide a limited amount oflegible information per display screen.

By contrast to real image displays, virtual image displays provide avirtual image, i.e., an image which, if a viewing surface werepositioned at the location of the virtual image, no image would beobserved by the eye. By definition, a virtual image can exist at alocation where no display surface exists. An example of a virtual imageis the image of fine print viewed through a magnifying glass.

Virtual image displays provide an image which appears to be larger thanthe source object from which the virtual image is formed. As a result,the size of the virtual image, as perceived by the user, is limited bythe magnification of the display system as opposed to the size of theelectronic display. This enables virtual image displays to provide theuser with a greater amount of legible information per display screenthan real image displays in the same space. It also enables a virtualimage display to be designed which provides the same amount ofinformation per screen as real image displays in a smaller space.

In general, virtual image displays include a source object which ismagnified by one or more optics to provide a virtual image along animage plane. The thickness of the display, i.e., the dimension of thedisplay that is perpendicular to the image plane of the virtual image,is dependent on the separation between the components of the displaysystem. A need exists for an inexpensive, compact virtual image displaysystem in which the separation between the components of the displaysystem are reduced so that the display system has a reduced thickness.

There are at least four parameters which relate to the ease of viewingan image produced by a virtual image display. The first parameter is thefar point which refers to the maximum distance from the eye that adisplay system can be held and have the eye still see the entire virtualimage. Display systems which provide a far point which is a shortdistance from the display are undesirable due to the inconvenience anddiscomfort associated with placing the eye in close proximity with thedisplay. A need therefore exists for a virtual image display systemwhich provides a sufficiently long far point so that the magnified imagecan be viewed at a comfortable and convenient range of distances fromthe display system.

The second parameter relating to the ease of viewing a virtual image isthe apparent angular width of the virtual image, commonly referred to asthe field of view of the virtual image. The full field of view isdefined as the ratio of the largest apparent dimension of the virtualimage to the apparent distance to the virtual image. It is generallyequivalent to the field of view for a real image display surface. A needexists for a virtual image display system which provides a wide field ofview.

The third parameter relating to the ease of viewing a virtual image isthe transverse distance that the eye may move with respect to theoptical system and still have the eye see the entire virtual imagethrough the display system. A need exists for a virtual image displaysystem which provides a long transverse distance through which the eyemay move with respect to the display system.

The fourth parameter relating to the ease of viewing a virtual image isillumination. In this regard, it is important that the virtual imageproduced be have a strong contrast ratio between illuminated andnon-illuminated pixels. A need therefore exists for a display systemwhich provides a bright virtual image. A further difficulty associatedwith virtual image displays is irregularities in the illumination of thesource object. A need therefore also exists for a display system whichprovides a virtual image having substantially uniform illuminationacross the image.

SUMMARY OF THE INVENTION

A compact virtual image display system is provided which has a reducedthickness, (the dimension perpendicular to the image plane of thevirtual image) without significantly reducing the field of view or eyerelief provided by the display system. Reducing the thickness of thedisplay system is accomplished using an immersed beam splitter, and inone embodiment, total internal reflection within the immersed beansplitter. In one embodiment, the display system produces a virtual imagewith enhanced brightness. In another embodiment, the display systemincludes at least one optic whose position is adjustable. Movement ofthe optic serves to focus the display system, thereby enabling thedisplay system to be used by a wide variety of users. Due to the designof the display system, only a very small adjustment in the positioningof one of the optics is needed in order to focus the display system.

According to the present invention, the display system includes animaging surface on which a source object is produced, a first opticalelement having a reflective function and a magnification function, asecond optical element having a magnification function and an immersedbeam splitting element positioned between the first and second opticalelements, the immersed beam splitting element including a beam splittersurrounded by one or more optically transparent materials having arefractive index greater than air. An illumination source projects lightonto the imaging surface to cause the source object to be projectedthrough the optically transparent material to the beam splitter. Thebeam splitter reflects the projected source object to the first opticalelement. The magnification function of the first optical elementmagnifies the projected source object and the reflection functionreflects a magnified first virtual image of the projected source objectto the beam splitter. The magnified first virtual image then traversesthe beam splitter and enters the second optical element which magnifiesthe first magnified virtual image to produce a compound magnifiedvirtual image of the source object.

In one embodiment, the beam splitter and first optical element havereflective surfaces, the angle between the reflective surfaces of thebeam splitter and the first optical element being between about 30 and45°, in another embodiment, 37°.

In one embodiment, the immersed beam splitting element has a backsurface facing the first optical element which causes light directed tothe back surface to be totally internally reflected within the opticallytransparent material when the angle of incidence of the light relativeto the back surface is less than an angle θ. According to thisembodiment, the source object is projected from the imaging surface tothe back surface of the immersed beam splitting element at less than ailangle θ such that the back surface reflects the light to the beamsplitter. By using total internal reflection in this manner, the anglebetween the reflective surfaces of the beam splitter and first opticalelement can be reduced to between about 10 and 30°. This enables theoverall thickness of the display system to be reduced.

In one embodiment, the optically transparent material used in theimmersed beam splitter has a refractive index of at least about 1.3.Examples of optically transparent materials include, but are not limitedto glass and plastics such as polymethyl methacrylate, polystyrene,polycarbonate, methyl methacrylate styrene, styrene acrylonitrile, andacrylonitrile butadiene styrene.

A polarization modifier may be used to improve the light efficiency ofthe display. According to this embodiment, the beam splitter isreflective to a first polarization of light and transmissive to a secondpolarization of light. In this embodiment, the light projected to thebeam splitter is of a first polarization. In this embodiment, thedisplay system also includes a polarization modifier positioned betweenthe beam splitter and the first optical element such that lighttraverses the polarization modifier after being reflected by the beamsplitter and after being reflected by the first optical element. Thepolarization modifier alters the light from a first polarization to asecond polarization so that the magnified first virtual image reflectedby the first optical element traverses the beam splitter. In oneembodiment, the polarization modifier is a quarter waveplate.

The imaging surface may be any surface on which a source object can beformed. For example, the imaging surface may be a microdisplay, i.e. anelectronically activated display which can produce a source object ofany type. Examples of microdisplays include, but are not limited toliquid crystal displays, spatial light modulators, gratings, mirrorlight valves and LED and FED arrays. In one embodiment, the microdisplayhas a display area of less than about 100 mm². The microdisplaygenerally includes an array of pixels which form a source object. In oneembodiment, the pixels have an area between about 49 μm² and about 225μm², in another embodiment, less than about 144 μm².

The imaging surface may also be a surface on which an image isprojected. For example, the imaging surface may be a light transmissiveor reflective screen on which a real image serving as the source objectcan be formed.

In one embodiment, a brighter display system is provided which includesa microdisplay having an array of pixels which can be modulated toeither reflect or scatter light and an optical grating which focuseslight scattered by the microdisplay onto the immersed beam splittingelement so that more of the scattered light is transmitted through theoptics to the user.

In a preferred embodiment, the display system also includes an eyeposition sensor for detecting the distance of a user's eye to thedisplay system. The eye position sensor provides a signal to the lighttype controlling mechanism which instructs the controlling mechanism asto the type of light to project to the light differentiating element.The eye position sensor may also be employed by the user to interactwith the display system, i.e., control the types of source objectsformed on the imaging surface as well as the operation of the devicehousing the display system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a display system in which an immersed beam splitteris used.

FIG. 2 illustrates a display system according to the present inventionin which total internal reflection off of the rear surface of the beamsplitting element is used to reduce the size of the display system.

FIG. 3 illustrates a display system in which an immersed beam splitteris not used and the microdisplay is positioned in the field of viewprovided by the optical system.

FIG. 4 illustrates a display system in which an immersed beam splitteris not used and the angle φ between the beam splitter and the opticalelements is increased.

FIGS. 5A-D illustrate a series of immersed beam splitting elementsformed of at least two different optically transparent materials.

FIG. 5A illustrates an immersed beam splitting element in which thesurface of the immersed beam splitting element which faces the firstoptical element is covered by a glass slide.

FIG. 5B illustrates an immersed beam splitting element in which thesurface of the immersed beam splitting element which faces the secondoptical element is covered by a glass slide.

FIG. 5C illustrates an immersed beam splitting element in which thesurface of the immersed beam splitting element which faces the firstoptical element is covered by a glass slide within which a beam splitterhas been incorporated.

FIG. 5D illustrates an immersed beam splitting element in which theentire surface of the element is covered by glass.

FIG. 6 illustrates a display system according to the present inventionin which a polarization discriminating beam splitter is used.

FIG. 7 illustrates a display system according to the present inventionwith a focusing mechanism.

FIG. 8 illustrates the reflective and scatter modes of a liquid crystaldisplay.

FIG. 9 illustrates a liquid crystal display with a grating for focusingscattered light into the optical system.

FIG. 10 illustrates a display system in which the imaging surface is ascreen on which a real image is formed.

FIG. 11 illustrates the electronics included within the display systemfor controlling the microdisplay.

FIG. 12 illustrates an eye position sensor system used in conjunctionwith the display system of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A virtual image display system is provided in which the thickness of thedisplay system is reduced, the thickness of the display referring to thedimension of the display perpendicular to the plane of the virtualimage. Reducing the thickness of the display system is accomplishedaccording to the present invention using an immersed beam splitter, andin one embodiment, total internal reflection within the immersed beamsplitter. According to the present invention, the thickness of thedisplay system is reduced without altering the other ergonomics of thedisplay system, i.e., without reducing the field of view or eye reliefprovided by the display system. In one embodiment, a display system isprovided which also produces a virtual image with enhanced brightness.In another embodiment, the display system includes at least one opticwhose position is adjustable. Movement of the optic serves to focus thedisplay system, thereby enabling the display system to be used by a widevariety of users. Due to the design of the display system, only a verysmall adjustment in the positioning of one of the optics is needed inorder to focus the display system.

The display system of the present invention is intended as aninexpensive component which may be incorporated into any electronicdevice in which a display system is used. In one embodiment, the displaysystem is designed for use in pocket-sized electronic devices. Examplesof such devices include, but are not limited to, portable computers,personal communicators, personal digital assistants, modems, pagers,video and camera viewfinders, mobile phones, television monitors andother hand held devices.

An embodiment of the display system of the present invention isillustrated in FIG. 1. As illustrated in the figure, the display system10 includes an imaging surface 12 on which a source object 14 is formed.In FIG. 1, the imaging surface 12 illustrated is a microdisplay whichforms the source object. Alternate embodiments of the imaging surfaceare described herein.

Light 18 from an illumination source 16 is projected onto the imagingsurface 12 to cause the source object to be projected into an immersedbeam splitting element 22 which includes a beam splitter 25 immersed inan optically transparent material 27. A portion of the light whichreaches the beam splitter 25 is reflected by the beam splitter 25 anddirected to first optical element 24.

First optical element 24 has a reflective function and a magnificationfunction. The image formed by the light projected onto the first opticalelement 24 is magnified by the magnification function of the element toform a magnified first virtual image 90. The light from the magnifiedfirst virtual image 90 is reflected by the reflective function of theelement. At least a portion of the light forming the magnified firstvirtual image 90 passes through the beam splitting element 22 to asecond optical element 26. The magnified first virtual image 90projected onto second optical element 26 is magnified by the secondoptical element 26 as the light traverses the second optical element 26to form a compound magnified virtual image 92 which is seen by observer28. The apparent position of the first magnified virtual image 90relative to the second optical element is approximately equal to or lessthan the focal length of the second optical element 26. The compoundmagnified virtual image 92 appears to be positioned further away thanthe first magnified virtual image 90, preferably at least 10 inchesaway. In one embodiment the compound magnified virtual image 92 is atleast about 6 feet away to provide for comfortable viewing.

In order to prevent stray light from interfering with the observer'sability to see the compound virtual image 92, in one embodiment, anantireflective coating is placed on the outside surface 34 of the secondoptical element 26.

FIG. 2 illustrates an alternate embodiment of the display system inwhich total internal reflection off the back surface 32 of the beamsplitting element 22 is used to further reduce the thickness of thedisplay. As illustrated in the figure, the display system 10 includes animaging surface 12 on which a source object 14 is formed. Illuminationsource 16 projects light 18 onto the imaging surface 12.

The light 18 projected onto the imaging surface 12 causes the sourceobject to be projected toward the back surface 32 of beam splittingelement 22. By angling the imaging surface 12 such that the lightcorresponding to the projected source object contacts the back surface32 of the beam splitting element 22 at an angle θ less than the angle atwhich total internal reflection occurs, the light forming the projectedsource object is reflected off the back surface 32 of the beam splittingelement 22 to beam splitter 25. The light is then reflected by the beamsplitter 25 and directed to first optical element 24.

The angle θ at which total internal reflection occurs depends on therefractive inidex material 27 used to form the immersed beam splittingelement 22, as well as whether a film is placed on the back surface 32of the beam splitting element 22. In one embodiment, angle θ is lessthan about 60.

As in the embodiment illustrated in FIG. 1, first optical element 24 hasa reflective function and a magnification function. The image formed bythe light projected onto first optical element 24 is magnified by themagnification function of the element to form a magnified first virtualimage 90. The light from the magnified first virtual image 90 isreflected by the reflective function of the element. At least a portionof the light forming the magnified first virtual image 90 passes throughthe beam splitting element 22 to a second optical element 26. Themagnified first virtual image 90 formed by the light projected ontosecond optical element 26 is magnified by the second optical element 26as the light traverses the second optical element 26 and is seen as acompound magnified virtual image 92 by the observer 28.

As illustrated in FIGS. 1 and 2, the beam splitter 25 is positionedrelative to the plane 30 of the first optical element 24 to direct lightfrom the imaging surface 12 onto the first optical element 24. Theapparent position of the first magnified virtual image relative to thesecond optical element is approximately equal to or less than the focallength of the second optical element. The compound magnified virtualimage 92 appears to be positioned further away than the first magnifiedvirtual image 90, preferably at least 10 inches away. In one embodimentthe compound magnified image 92 is at least about 6 feet away to providefor comfortable viewing.

The angle φ between the beam splitter 25 and plane 30 limits how closethe first and second optical elements 24, 26 may be positioned relativeto each other (shown in FIGS. 1 and 2 as distance t) and hence limitsthe overall thickness T of the display system.

According to the present invention, the separation required between thefirst and second optical elements 24, 26 is reduced by using immersedbeam splitter 22, i.e., a beam splitter 25 which is contained within oneor more optically transparent materials 27 which have a refractive indexgreater than air. By using an immersed beam splitter, light passesbetween the imaging surface 12 and the beamsplitting element 22 througha medium having a higher refractive index. The higher refractive indexmedium causes the source object projected from the imaging surface 12 tobe magnified in comparison to an image of the source object which passesbetween the imaging surface 12 and the beamsplitting element 22 throughair. The magnification performed by the beamsplitting element 22 enablesa smaller beamsplitting element 22 to be used in the display system,which, in turn, enables the beamsplitting element 22 to be positioned ata tighter angle φ.

By reducing the angle φ at which the beamsplitting element 22 ispositioned, the distance t between the first and second optical elements24, 26 is reduced.

In one embodiment of the display, as illustrated in FIG. 1, the angle φbetween the beam splitter 25 and plane 30 is between about 30 and 45°,in another embodiment, 37°. In another embodiment of the display systemwhich utilizes total internal reflection within beamsplitting element22, as illustrated in FIG. 2, the angle φ between the beam splitter 25and plane 30 is between about 10 and 30°, in another embodiment, 25°.Thus, through the total internal reflection employed in the embodimentillustrated in FIG. 2, the angle φ between the beam splitter 25 andplane 30 and hence the separation required between the first and secondoptical elements 24, 26 is reduced.

The use of an immersed beam splitter also enables a smaller imagingsurface 12 to be used and/or for the imaging surface 12 to be positionedfurther away from the beam splitter 25 than if the source object wereprojected from the imaging surface 12 to the beam splitter 25 throughair. This is important to the compact design of the display system ofthe present invention since it enables the imaging surface to bepositioned out from between optical elements 24, 26 and thus out of thefield of view provided by the optical system.

FIGS. 3-4 illustrate display systems having a layout similar to thedisplay illustrated in FIG. 1 where the beam splitter 25 is not immersedin an optically transparent material 27 having a refractive indexgreater than air. As illustrated in FIG. 3, in order to create a firstvirtual image 90 of the source object 14 at a position approximately f₁behind the second optical element 26, (f₁ =the focal length of firstoptical element 24), the imaging surface must be moved closer to thefirst optical element 24 by a factor of 1/n_(imm) where n_(imm) is theindex of refraction of the immersing medium. This causes the imagingsurface 12 to be positioned between optical elements 24, 26 and withinthe field of view provided by the optical system.

As illustrated in FIG. 4, when the beam splitter 25 is not immersed inan optically transparent material 27 having a refractive index greaterthan air, and when the imaging surface 12 is not brought closer to thefirst optical element 24, the magnification of the compound virtualimage 92 relative to when an immersed beam splitter 22 is used isreduced by a factor of 1/n_(imm) where 1/n_(imm) is the refractive indexof the immersing medium. In order to produce a compound virtual image 92with an equivalent magnification relative to when an immersed beamsplitter is used, it is necessary to use a larger imaging surface. This,however, has the disadvantage that it requires increasing the angle φbetween the beam splitter 25 and the plane 30 of the first opticalelement, as illustrated in FIG. 4. This causes the separation t betweenoptical elements 24, 26 to increase and thus the overall thickness T ofthe display system to increase relative to the display systemillustrated in FIG. 1.

A variety of optically transparent materials may be used as theimmersing medium 27 in beam splitting element 22 including glass andmolded plastics, such as polymethyl methacrylate, polystyrene,polycarbonate, methyl methacrylate styrene, styrene acrylonitrile, andacrylonitrile butadiene styrene. In one embodiment, the opticallytransparent material 27 has refractive index of at least about 1.3.

The beam splitter 25 acts as both a reflective and transmissive opticalelement in the display system. In one embodiment, the beam splitter 25simply reflects and transmits portions of the light directed to the beamsplitter 25, typically approximately 50% reflective and 50%transmissive. These types of beam splitters should have a coating with abroader bandwidth than the illumination source. Types of coatings whichmay be used with this type of beam splitter include, but are not limitedto, dielectric, metal/dielectric and metal coatings. Hybridmetal/dielectric coatings are preferred because they provide moderatelight absorption with little polarization sensitivity. These coatingsalso show relative insensitivity to the angle of incidence.

In another embodiment, illustrated in FIGS. 5A-D, the beam splittingelement 22 may include a beam splitter 25 immersed in a combination oftwo optically transparent materials which each have an index ofrefraction greater than air. In general, the first optically transparentmaterial 51 is preferably well adapted to be coated with one of avariety of optical coatings which one of ordinary skill may wish toapplied to one or more surfaces of the immersed beam splitting element.Glass is particularly preferably as the first optically transparentmaterial due to the reduced cost associated with coating glass ascompared to other materials, such as plastic.

In order to reduce the overall cost of beam splitting element, thesecond optically transparent material 61 is preferably a lower costmaterial than the first optically transparent material 51. For example,plastic or transparent liquids may be used as the second opticallytransparent material 61. The second optically transparent material mayalso be used to form the surface interfaces which are not formed of thefirst optically transparent material. By using these materials incombination, the cost of the immersed beam splitting element can beminimized.

According to this embodiment, a first optically transparent material 51covers one or more of the surfaces of the immersed beam splitter 25through which light within the display system enters and/or exits theimmersed beam splitter 25. These surfaces include, but are not limitedto the beam splitter surface 53 opposite the illumination source 16, thebeam splitter surface 55 opposite the imaging surface 12, the beamsplitter surface 57 opposite the first optical element 24, and the beamsplitter surface 59 opposite the second optical element 26.

A variety of methodologies are known to one of ordinary skill in the artfor forming a beam splitter having a combination of two opticallytransparent materials. For example, the first optically transparentmaterial may be a glass slide which is glued to one or more surfaces ofthe second optically transparent material. The beam splitter may beincorporated into either the first or second optically transparentmaterial in the beam splitting element.

Several embodiments of immersed beam splitting elements having acombination of two different optically transparent materials areillustrated in FIGS. 5A-D. It should be understood that these embodimentare not intended to be limiting as to the variety of immersed beamsplitting elements that may be constructed using a combination of two ormore optically transparent materials.

Illustrated in FIG. 5A is an immersed beam splitting element 22 formedof plastic 61 in which a glass slide 51 is positioned over surface 57 ofthe immersed beam splitting element 22 which faces the first opticalelement. Illustrated in FIG. 50 is an immersed beam splitting element 22formed of plastic 61 in which a glass slide 51 is positioned oversurface 59 of the immersed beam splitting element 22 which faces thesecond optical element 26. Illustrated in FIG. 5C is an immersed beamsplitting element 22 formed of plastic 61 in which a glass slide 51 ispositioned over surface 59 of the immersed beam splitting element 22which faces the first optical element 24. In this embodiment, the beamsplitter 25 is incorporated into the glass slide 51 as opposed to theplastic 61. Illustrated in FIG. 5D is an immersed beam splitting element22 in which the entire surface of the element is covered by glass 51. Inthis embodiment, a volume is defined within the glass. Because the glasssubstantially surrounds the volume, this immersed beam splitting elementdesign is well adapted for using a liquid as the second opticallytransparent material 61.

In another embodiment, a beam splitter 25 is used whose reflective andtransmissive properties are polarization dependent, i.e., the beamsplitter 25 is more highly reflective at one polarization of light andmore highly transmissive at another polarization of light. When a 50:50beam splitter is used, 75% of the light is lost in the display system ofthe present invention. However, by using a polarization dependent beamsplitter in combination with a polarization modifier, light can bedirected within the optical system without the loss of light associatedwith non-discriminant beam splitters.

As illustrated in FIG. 6, a polarization modifier 29, such as a quarterwaveplate, is positioned between the beam splitter 25 and the reflectivefirst optical element 24 in this embodiment. The polarization modifier29 serves to alter the polarization of the light which traverses thepolarization modifier before and after being reflected off the firstoptical element 24. By using the polarization modifier 29, polarizedlight from the microdisplay 12 is reflected by the beam splitter 25,altered as it traverses the polarization modifier 29, reflected by thefirst optical element 24 and altered again by the polarization modifier29 so that the light traverses the beam splitter 25 after beingreflected off the first optical element 24.

The position at which an individual is best able to focus on an objectis dependent on that individual's vision. For example, some individualsare far sighted (hyperopic) and are better able to visualize image whichappear to be positioned at a far distance. By contrast, otherindividuals are near sighted (myopic) and are better able to visualizeimage which appear to be positioned close to the eye. In order toaccommodate these differences in the vision capabilities of potentialusers, an embodiment of the display system is provided in which theapparent position of the compound virtual image to the user can bevaried. Adjustment to the apparent position of the compound virtualimage to the user, also referred to herein as focusing, is accomplishedby a mechanism which moves the first optical element 24 relative to thebeam splitter 25.

As illustrated in FIG. 7, the first optical element 24 and/or secondoptical element 26 can be moved relative to the beam splitter 25 by afocusing mechanism 67. Movement of the first optical element 24 relativeto the beam splitter 25 causes the compound magnified virtual image 92to appear at different distances from the observer's eye. For example,the first optical element may be moved to make the compound magnifiedvirtual image 92 appear at a position within several inches of theobserver's eye to a position numerous feet from the observers eye.Movement of the second optical element 26 relative to the beam splitter25 causes the first magnified virtual image 90 to appear to be atdifferent distances relative to the observer. Due to the overall designof the optical system, this wide range focusing of the display systemcan be accomplished by small movements of the first optical element 24and/or second optical element 26 relative to the beam splitter 25.

Table 1 provides data which indicates the perceived distance between acompound magnified virtual image (d_(cmvi)) and an observer for a seriesof separation distances (d_(sep)) between the source object and firstoptical element. In the display system upon which this data is based,the first optical element has a radius of 60 mm, the second opticalelement has a focal length of 60 mm and the index of refraction of theimmersion medium is 1.5. As shown by the data in Table 1, at aseparation distance of 27.5 mm, the compound magnified virtual imageappears about 1.5 feet from the observer. However, by moving the firstoptical element 1.4 mm to a separation distance of 28.9 mm, the compoundmagnified-virual image appears to be about 79 feet from the observer.Thus, as can be seen from this data, only a s mall movement of the firstoptical element relative to the beam splitter causes the compoundmagnified virtual image to appear to move a significant distancerelative to the observerp's eye.

                  TABLE 1    ______________________________________           D.sub.sep                  D.sub.cmvi    ______________________________________           27.5 mm                  1.53 ft           27.7 mm                  1.8 ft           27.9 mm                  2.1 ft           28.1 mm                  2.7 ft           28.3 mm                  3.5 ft           28.5 mm                  5.2 ft           28.7 mm                  9.8 ft           28.9 mm                   79 ft    ______________________________________     D.sub.cmvi distance between a compound magnified virtual image and an     observer     D.sub.sep separation distances between the source object and first optica     element

The imaging surface 12 employed in the display system is used as asource of a source object which is projected to the beam splittingelement 22. In its broadest sense, the imaging surface may be anysurface or device on which a real image may be formed or generated. Inone embodiment, the imaging surface is a microdisplay. The microdisplaymay be any electronically activated display which can produce a sourceobject of any type. For example, the microdisplay may be a liquidcrystal display, a spatial light modulator, a grating, a mirror lightvalve or an LED array. More specific examples of microdisplays which maybe used in the display system of the present invention include, but arenot limited to light transmissive liquid crystal displays, cholestericliquid crystal displays, PSC liquid crystal displays and spatial lightmodulators. In a preferred embodiment, the microdisplay is a lighttransmissive liquid crystal on silicon display.

According to one embodiment of the invention, the microdisplay has adisplay area of less than about 100 mm². In one embodiment, themicrodisplay has pixels whose size is between about 49 μm² and about 225μm². In another embodiment, the pixels are less than about 144 μm². Atthese pixel and display size ranges, the microdisplay has a comparablenumber of pixels to that of a high resolution computer monitor.

Any illumination source which provides light in the visible range may beused in the display system of the present invention. Examples ofillumination sources which may be used in the present invention include,but are not limited to incandescent lamps, lasers and light emittingdiodes.

Microdisplays may generally be divided into two categories, reflectiveand light transmissive displays. Reflective microdisplays form a sourceobject by modulating which pixels reflect or scatter incident light. Bycontrast, light transmissive displays form a source object by modulatingwhich pixels are transmissive or opaque. Accordingly, when themicrodisplay is a light transmissive microdisplay, illumination isprovided from the rear of the microdisplay. When the microdisplay is areflective, illumination is provided so that the illumination isreflected off the imaging surface of the microdisplay toward the beamsplitting element.

In one embodiment, the illumination source 16used in the display systemis designed in combination with the microdisplay such that light fromthe illumination source uniformly illuminates the source object formedby the microdisplay. By providing uniform illumination to themicrodisplay, a virtual image is produced by the display system which isuniformly illuminated as the user's eye is moved within the field ofview.

Depending on the microdisplay employed in the display system, differentforms of illumination are particularly well adapted for providinguniform illumination. For example, for transmissive liquid crystaldisplays and cholesteric liquid crystal displays, it is preferred thatthe display be backlit on axis with the beam splitting element. Forreflective special light modulators and PSC liquid crystal displays, itis preferred that the illumination be provided to the face of thedisplay off axis, for example by a light emitting diode.

An illustration of an off-axis illumination system used in combinationwith a liquid crystal silicon display is provided in FIG. 8. Asillustrated in the figure, the liquid crystal display includes pixel 37which can be modulated between two modes 38, 40. The first mode 38 is areflective mode where light incident on the liquid crystal displaytraverses the pixel and is reflected off of a reflective surface 43underlying the pixel 37 in a direction away from the optical system 41.Light which is reflected by a pixel in a reflective mode does not enterthe optical system 41 and reach the user's eye. As a result, pixels in areflective mode appear dark 42 to the user. The second mode 40 is ascatter mode where light incident on the liquid crystal display isscattered by the display in all directions. A portion of the light thatis scattered in the scatter mode enters the optical system 41 andreaches the user's eye. As a result, pixels in the scatter mode appearbright 44 to the user.

FIG. 9 illustrates an alternate embodiment of the display system inwhich an optical grating is used to focus light scattered by amicrodisplay pixel into the field of view of the display. As illustratedin FIG. 9, the liquid crystal display includes an optical grating 39covering the reflective surface 43 underlying each pixel 37. The grating39 serves to focus light which traverses the microdisplay to thereflective surface in the direction of the optical system 41. As aresult, more of the light scattered by a pixel in a scatter mode isdirected through the optical system to the user. Since more scatteredlight reaches the user when a grating 39 is used, the images produced bythe display system of FIG. 9 are brighter than images produced by thedisplay system illustrated in FIG. 8.

The imaging surface may also be a surface on which a real image isformed. As illustrated in FIG. 10, the display system includes a realimage generating mechanism 73 which projects an image reflect off mirror75 onto the imaging surface 12 which serves as the source object 14 forthe display system. The real image generating mechanism 73 may include,for example, a microdisplay as described above, which forms the image,as well as optics which project and, optionally, magnify the image.

As illustrated in FIG. 10, the imaging surface 12 may be a lighttransmissive screen onto which a real image (the source object) isformed. It should also be understood that the imaging surface may be areflective screen onto which a real image (the source object) is formedwhere light corresponding to the source object is reflected toward thebeam splitting element 22.

The first and second optical elements 24, 26 used in the display systemof the present invention are selected such that the optics arecomplementary to each other so that they cancel out many of theaberrations introduced by each optic independently. As a result, thecombined magnified virtual image produced by the first and second opticsis substantially free of aberrations. Depending on the opticalcorrection which is needed, the first and second optical elements 24, 26may have spherical or aspherical surfaces. Examples of aberrations whichcan be corrected by the pairing of first and second optical elements 24,26 include, but are not limited to field curvature, coma, astigmatismand distortion.

The first and second optical elements 24, 26 may be made from a varietyof materials including glass and plastic. Due to cost considerations,the optics are preferably formed of a molded plastic. A variety ofdifferent optical designs may be employed in the first and secondoptical elements. For example, the optical elements may each be formedof a single optic or multiple optical elements. The magnificationfunction of each optic may be performed using a variety of differentlens designs, including, but not limited to a fresnel lens, aholographic lens or a diffractive optical lens. In one embodiment, a HRcoating at illumination wavelengths is used on the first optical elementto provide the optic with a reflective function. In one embodiment, thefirst optical element 24 provides magnification by a factor of betweenabout 5 and 10, in another embodiment by a factor of about 6. In oneembodiment, the second optical element 26 provides magnification by afactor of between about 2 and 5, in another embodiment by a factor ofabout 3.

FIG. 11 illustrates the electronics included within the display systemfor controlling a microdisplay serving as the imaging surface. Asillustrated in FIG. 11, the microdisplay 12 is connected to the controldevice 56 and conveys electrical signals to the microdisplay whichcontrols the source object generated. The control device 56 may also beconnected to a logic processor 58 which is also connected to externalmemory 60 which may be controlled through an external data source 62. Ina preferred embodiment, the pixels of the microdisplay 12 are addressedby binary inputs which cause the pixels to be modulated between on andoff states. The microdisplay 12 is connected to a control device 56 suchas a character generator with timing and decoding logic. An example of acontrol device 56 suitable for use in systems according to the presentinvention is illustrated in FIG. 11. A commercially available example ofa suitable control device is the VGA Controller, Product No.SPC8108F_(oc) sold by SMOS.

The control device 56 is controlled by a processor 58 which manipulatesthe data from the external memory 60. The external memory receives theinformation from the external data source 62 such as an external radioreceiver. The external data source 62 could also be an infrared diodedata link to another computer, LAN system, or any other device capableof sending analog or digital communications. In a preferred embodiment,the external memory 60 and external data source 62 is a separate PCMCIAcard which can be connected to a computer or communication system.

In a further, preferred embodiment, the display system includes an eyeposition sensor system. The sensor system enables the user to use his orher eye to interact with a control device. The eye position sensorsystem can be used by the user to control the images provided by thedisplay. The eye position sensor system can also be used to control avariety of functions performed by the electronic device, for example,directing a printer to print a document or directing a facsimile machineto send a message. According to this embodiment, the position of theobserver's eye is detected and used, much like a cursor, to interactwith the controlling device to control the source object produced by themicrodisplay.

Devices, such as eye trackers and occulometers, for detecting theposition of the eye, are well known in the art. For example, suitabledevices which may be used in conjunction with the virtual mouse sensorsystem include the device described in U.S. Pat. No. 4,513,317 which isincorporated herein by reference.

In one embodiment, illustrated in FIG. 12, the eye position sensorsystem includes a light emitting diode (LED) 72 which may be positionedaround the perimeter of the second optical element 26 for providingillumination in the direction of the user's eye 28. The illumination ispreferably in the infrared region. The eye position sensor system alsoincludes a detector array 76 for detecting reflections of theillumination from the LED 72 off of the retina 78 of the eye 28, thereflections serving to indicate the position of the eye 28 as well asthe direction of the eye. The eye position sensor system may alsoinclude a control mechanism 80 which the observer uses in combinationwith the detector array 76 to interact with the control device 56 tocontrol the source object 14 produced by the microdisplay 12.

The control mechanism 80 may be, for example, a button which the userdepresses to indicate that the observer is looking at a selected item,such as a computer software icon. The control mechanism 80 may also be atiming mechanism which determines that the user has selected an itembased on the amount of time that the observer is looking in a particulardirection.

The foregoing description of preferred embodiments of the presentinvention has been provided for the purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise forms disclosed. Obviously, many modificationsand variations will be apparent to practitioners skilled in this art.The embodiments were chosen and described in order to best explain theprinciples of the invention and its practical application, therebyenabling others skilled in the art to understand the invention forvarious embodiments and with various modifications as are suited to theparticular use contemplated. It is intended that the scope of theinvention be defined by the following claims and their equivalents.

What is claimed is:
 1. An optical method comprising:(a) directing asource object within a first optical element to a surface of the firstoptical element where the source object is internally reflected; (b)directing the source object to a beamsplitter which reflects the sourceobject; (c) directing the source object to a second optical elementwhich magnifies and reflects the source object to form a magnifiedvirtual image of the source object; (d) directing the magnified virtualimage through the beamsplitter; (e) directing the magnified virtualimage to a third optical element which magnifies the magnified virtualimage to produce a compound magnified virtual image of the sourceobject.
 2. The optical method according to claim 1, the method furtherincluding forming the source object using a microdisplay.
 3. The opticalmethod according to claim 2, wherein the microdisplay has a display areaof less than about 10 mm×10 mm.
 4. The optical method according to claim2, wherein the microdisplay has pixels having a size between about 7μm×7 μm and about 15 μm×15 μm.
 5. The optical method according to claim2, wherein the microdisplay has pixels having a size less than about 12μm×12 μm.
 6. The optical method according to claim 1, wherein the firstoptical element is formed of an optically transparent material which hasa refractive index of at least about 1.3.
 7. The optical methodaccording to claim 1, wherein the source object is directed to theinternally reflecting surface of the first optical element at an angleof at least about 30°.
 8. An optical method comprising:(a) directing asource object within a first optical element to a surface of the firstoptical element where the source object is internally reflected; (b)directing the source object within the first optical element to abeamsplitter which reflects the source object; (c) directing the sourceobject within the first optical element to a surface of the firstoptical element which magnifies and reflects the source object to form amagnified virtual image of the source object; (d) directing themagnified virtual image through the beamsplitter out of the firstoptical element; (e) directing the magnified virtual image to a secondoptical element which magnifies the magnified virtual image to produce acompound magnified virtual image of the source object.
 9. The opticalmethod according to claim 8, the method further including forming thesource object using a microdisplay.
 10. The optical method according toclaim 9, wherein the microdisplay has a display area of less than about10 mm×10 mm.
 11. The optical method according to claim 9, wherein themicrodisplay has pixels having a size between about 7 μm×7 μm and about15 μm×15 μm.
 12. The optical method according to claim 9, wherein themicrodisplay has pixels having a size less than about 12 μm×12 μm. 13.The optical method according to claim 8, wherein the first opticalelement is formed of an optically transparent material which has arefractive index of at least about 1.3.
 14. The optical method accordingto claim 8, wherein the source object is directed to the internallyreflecting surface of the first optical element at an angle of at leastabout 30°.